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    mathstuff01's Avatar
    mathstuff01 Posts: 1, Reputation: 1
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    #1

    Jan 21, 2009, 02:08 PM
    Sequences and Series Gr. 11 math- arithmetic sequence
    The 6th term of an arithmetic sequence is x while the 11th term is y. Find the first two terms

    I know that it starts by x= a +5d and y= a + 10d
    how I don't know where to go from there. Thanks
    ebaines's Avatar
    ebaines Posts: 12,131, Reputation: 1307
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    #2

    Jan 26, 2009, 08:42 AM

    You're almost there. Subtract x from y and you get y - x = 5d, so therefore d = (y-x)/5. Now subtact 5d from x to find a - you now have values for both "a" and "d" in terms of "x" and "y."
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    pabeer Posts: 1, Reputation: 1
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    #3

    Aug 25, 2010, 07:30 AM
    In an arithemetic series, t1 =6 and S9=108. Find the common difference and sum of the first 20 terms.
    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
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    #4

    Aug 25, 2010, 08:34 AM

    Do you know the formulae to use?





    or



    Plug in what you know. One of these will give you an equation which you'll need to solve.

    Then use the second formula to find the sum of the first 20 terms, by taking n = 20.
    Abhibarua's Avatar
    Abhibarua Posts: 2, Reputation: 1
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    #5

    Dec 30, 2011, 10:07 PM
    The nth term of an AP is a and the q term is b. Prove the sum of the first (p+q) term is p+q/2(a+b+a-b/p-q)
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